Powder flowability can be defined as the property of the powder to flow evenly from the top to the bottom of the hopper and then to the dosage, compact and crush chamber under the influence of gravity force and other forces. Poorly flowing powders present many difficulties in the pharmaceutical industry both in compressed tablet manufacturing and in capsule filling operations. A free-flowing powder on the other hand offers many advantages which, for instance in case of the manufacture of compressed tablet, can be summarized as follows:
(a) The powder easily flows in the hopper without forming air pockets;
(b) The die cavity is filled more efficiently, and this is reflected in an increase of the mean tablet weight and a decrease of the coefficient of variation, due to lack of low dosing;
(c) As a consequence of the uniform tablet weights and uniform doses of active ingredients, also other parameters of the finished tablets, such as hardness, friability, disintegrating time, dissolution test and plasma levels are reproducible;
(d) Another consequence of the more efficient die cavity filling will be reflected in the uniformity of the compressing power and therefore less wear of the machine;
(e) Since a free-flowing powder also has a good permeability, it will result in an easy ejection of air during the compression and therefore fewer defective tablets due to capping or splitting;
(f) High production rate allowed by the high flow rate of a free-flowing powder.
In view of this, many efforts have been made recently to overcome problems created by poor-flow properties by studying different formulations and applying different manufacturing techniques. At the same time many attempts have been made to establish experimental procedures having practical industrial significance which can really measure or assess the intrinsic flowability of the powder to be incapsulated or compressed. A number of authors identified the powder flowability with the interparticulate friction, of which the "angle of repose" is a manifestation (E. Nelson - J. Am. Pharm. Assoc. Sci. Ed. 44, No. 7, 435-437 (1955)) and at least four practical methods of measuring the angle of repose have been developed which are described and compared by David Train in J. Pharm. Pharmacol. 10, 127T to 135T (1958).
Basically, according to the "angle of repose" method the powdered material is allowed to fall freely through an orifice onto a flat surface to form a conical pile of the deposited material and the angle between the surface of the cone and the horizontal plane is known as the angle of repose. A high angle would indicate a poorly flowing material whereas a low angle would indicate good flow.
Timed delivery through an orifice is an other method often used for evaluating the flowability of materials. A stop watch is usually used to either time a certain weights of powder flowing through the orifice or to close the orifice after a given time so that the powder flowing through in that time period can be weighed.
Even though the angle of repose measurements and the timed delivery technique give results somehow relatable to flow properties of the material, in the best of cases reproducibility is quite poor.
An explanation for the unsuccess of these tests is that the outlined tests did not hit the focal point of the problem. In particular, it is wrong to identify the flowability with the interparticle friction, as though powders were glass or sand balls.
The truth is that the parameters which determine the flowability of the powder are numerous and with contrasting and interdependent influence: granulometric, "fines", unit surface, shape of particles, actual density, apparent density, porosity, air permeability through powder, electrostatic charge, humidity, dwell, cohesion strengths (such as London, hydrogen, etc.)
Ausburger and Shangraw (J. Pharm. Sci, 55, No. 4, 418-423 (1966)), attempted to evaluate and compare the free flowing properties of powders using the weight and weight variation of the finished tablets as the measured parameter. It was felt in fact that the weight variation of both capsules and tablets is directly dependent upon the reproducibility of powder flow into a fixed volume receptable (which may be a tablet die cavity or a capsule shell) and that good precision, which reflects in a higher tablet weight and a lower coefficient of variation, can only be obtained when the powder to be filled has a good flowability. Even though this method can be conveniently used in routine quality control tests, it is tedious, time consuming and not practical at the preliminary product development stage mainly because it requires a large quantity of drug.
A further method for determining the free flow properties of a powder has been described in DEGUSSA Schriftenreihe, Anwendungstechnik Pigmente Nr. 31 (Wolfgang Hanau (Main)) pages 6 to 8 and is based on how the powder runs through sand timer-like funnels with varying orifices. The equipment there described consists of a series of five glass funnels with orifice diameters of 2.5, 5, 8, 12, and 18 mm, and the powder flowability is ranked as outstanding, very good, good, acceptable or poor depending on the diameter of the orifice which the powder can still pass through. This equipment however only gives a rough estimation of the flowability of a powder, and it does not provide reproductive and precise enough results to be relied upon in the pilot or industrial plants.